Humboldt says that the regularity of the hourly variations of the magnetic needle and the atmospheric pressure is undisturbed on earthquake days within the tropics. In seventeen observations, which I made during earthquakes in Lima with a good Lefevre barometer, I found, in fifteen instances, the position of the mercury quite unaltered. On one occasion, shortly before a commotion, I observed it 2·4 lines lower than it had been two hours before. Another time, I observed, also on the approach of the shock and during the twelve following hours, a remarkable rising and sinking in the column. During these observations the atmosphere was entirely tranquil.
Atmospheric phenomena are frequent, but not infallible prognostics of an earthquake. I have known individuals in Lima, natives of the coast, who were seldom wrong in predicting an earthquake, from their observation of the atmosphere. In many places great meteors have been seen before the commotion. Before the dreadful earthquake of 1746, there were seen fiery vapors (_exhalaciones encendidas_) rising out of the earth. On the island of San Lorenzo these phenomena were particularly remarked.
Many persons have an obscure perception--a foreboding, which is to them always indicative of an approaching earthquake. They experience a feeling of anxiety and restlessness, a pressure of the breast, as if an immense weight were laid on it. A momentary shudder pervades the whole frame, or there is a sudden trembling of the limbs. I, myself, have several times experienced this foreboding, and there can scarcely be a more painful sensation. It is felt with particular severity by those who have already had the misfortune to have been exposed to the dangers of an earthquake.
I will here only briefly mention the celebrated earthquake of 1746, as all its details are fully described in many publications. The reader need scarcely be reminded that it happened on the 28th of October, the day of St. Simon and St. Jude. During the night, between ten and eleven o'clock, the earth having begun to tremble, a loud howling was heard, and, in a few minutes, Lima became a heap of ruins. The first shock was so great, that the town was almost completely destroyed by it. Of more than 3000 houses, only twenty-one remained. Still more horrible was the destruction in the harbor of Callao. The movement of the earth had scarcely been felt there, when the sea, with frightful roaring, rushed over the shore, and submerged the whole town with its inhabitants. Five thousand persons were instantly buried beneath the waves. The Spanish corvette San Fermin, which lay at anchor in the port, was thrown over the walls of the fortress. A cross still marks the place where the stern of the vessel fell. Three merchant vessels, heavily laden, suffered the same fate. The other ships which were at anchor, nineteen in number, were sunk. The number of lives sacrificed by this earthquake has not been, with perfect accuracy, recorded.[38] Humboldt, in his Cosmos, mentions that during this earthquake a noise like subterraneous thunder was heard at Truxillo, eighty-five leagues north of Callao. It was first observed a quarter of an hour after the commotion occurred at Lima, but there was no trembling of the earth. According to the old chronicle writers, the earthquake of 1630 was more disastrous.